Membrane cofactor protein (MCP or CD46) is a 51-68 kDa integral membrane glycoprotein. It is present on all circulating cells, except erythrocytes, and on nearly all other cell types examined so far including haematopoietic, epithelial and endothelial cells. MCP was known earlier as a trophoblast/leucocyte common antigen (TLX). It has an overall structure very similar to DAF, containing four SCRs with three glycosylation sites. There are two differently glycosylated forms of MCP (58-68 kDa and 51-59 kDa). Altogether, four different isoforms are expressed by individual cells. The expression level of the various isoforms may vary between different tissues and cell types. No deficiencies of MCP have been described yet.
MCP binds to C3b and acts as a cofactor for factor I in mediating cleavage of C3b. MCP has also weak cofactor activity for the cleavage of C4b. The binding site of MCP for C3b is located within the third and fourth SCRs most adjacent to the cell membrane. The C-terminus of the protein contains 70 amino acids rich in serine, threonine and proline residues (STP-rich region). The STP-rich and heavily glycosylated C-terminal region probably directs the protein away from the cell membrane and protects it from proteolysis.
Very importantly, MCP has been identified as the cellular receptor for the measles virus. Monoclonal antibodies against MCP have been shown to prevent infection of human cells with the measles virus.
1. Liszewski M K, et al. (1991). Membrane cofactor protein (MCP or CD46): newest member of the regulators of complement activation gene cluster. Annual review of immunology, 9(1), 431-455.
2.Seya T, et al. (1990). Complement-mediated tumor cell damage induced by antibodies against membrane cofactor protein (MCP, CD46). The Journal of experimental medicine, 172(6), 1673-1680.